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Arceus has the Ability Multitype, which changes its type and appearance depending on the Plate it is holding. Likely as a precaution for Arceus hacked to be ???-type, a ???-type sprite of Arceus is present in Pokémon Diamond, Pearl, Platinum, HeartGold, and SoulSilver; however, it is never otherwise used without hacking.

Arceus is not meant to be obtained as ??? Type, the only reason why a sprite existed for it was in case someone hacked their Arceus. It is not seen anywhere else, so you will never see a ??? Arceus except through hacks.

??? isn't meant to be a type of obtainable Pokemon, it is only used to make eggs and the move Curse seem mysterious.